U.S. Pat. No. 4,611,219 discloses an ink-jet print head having at least one group of aligned expulsion chambers.
Each chamber contains a transducer for causing expulsion of the ink simultaneously from two nozzles.
All the nozzles are aligned in a single row in the direction of alignment of the chambers, forming a line of nozzles designed in particular to print an entire line at a time and hence obtain printing of a complete page with a single scanning movement.
A head of this type, with all the nozzles aligned in a single row, is able to print, whenever activated, at the most a continuous, but very thin line with a width equal to the dimension of each dot.
Therefore, in order to print characters or graphic symbols with a width much greater than the dimension of each dot, several passing movements are required, hence reducing the printing speed.
U.S. Pat. Nos. 4,542,389 and 4,550,326 disclose print heads of the type mentioned above, in which each expulsion chamber has associated with it several openings. Of these openings only one constitutes the active nozzle for expelling drops of ink, being arranged in the region of the heating resistor.
Additional openings communicating with the same pressure chamber are used in order to drain an excess of ink dispersed by the active nozzle or in order to neutralize reflex pressure pulses capable of influencing negatively the operation of active nozzles associated with other adjacent expulsion chambers.
These additional openings or orifices have a completely passive function since they do not expel drops of ink, not being associated with any heating resistor.
In a conventional ink-jet print head, for example of the type described in the U.S. Pat. No. 4,550,326 already referred to, the single active nozzle of a given expulsion chamber is normally dimensioned so as to expel drops of ink, the volume of which depends substantially on the energy supplied by the resistor and its dimensions.
Usually the active nozzle is constructed with a diameter more or less the same as the dimension of a side of the associated resistor which is generally square in shape, a dimension equal, for example, to about 40 to 60 .mu.m in the case of a printing resolution of 300 dots per inch.
Therefore, when particularly dense information or very intense images must be printed with this head, for example on a sheet of paper, the large quantity of ink deposited on the paper through the nozzle requires a given amount of time in order to dry, a time which in many cases is too long compared to the printing speed of the head.
Moreover the characteristic restoration time for the meniscus of a large-size nozzle, as referred to above, is fairly long and such that it limits the expulsion frequency of the drops to fairly low values.
Furthermore it is true that, according to a first approximation and with all other parameters, such as for example the characteristics of the ink, being equal, the expulsion frequency depends inversely on the volume of the drops expelled.
However, if drops with a small volume, for example a volume less than 80-90 pl, are used, again in the case of a printing resolution of 300 dots per inch, there is a deterioration in the print parameters such as, for example, the optical density and the quality of the edges of graphic symbols.
This limitation penalizes considerably ink-jet heads, compared to other faster dot printing methods, for example laser printing.
Furthermore, this head also has the following drawbacks:
unsatisfactory optical density, unless large quantities of ink are used to obtain intense colours; PA1 non-linear shades of grey, when there is a variation in the number of dots deposited; PA1 poor linearity of the edges of elongated impressions, for example the letters I, L, etc.
The optical density is considered unsatisfactory for the following reason: if a single nozzle is used, the impression of a drop of ink on the paper is substantially circular, so that the arrangement, next to one another, of several impressions which are mutually tangent and circular, i.e. with a diameter equal to the printing pitch, results, as is known, in a white zone, not covered by ink, inside each group of four adjacent impressions.
In order to eliminate these white zones, the impression of each dot must be widened by varying the moistness characteristics of the ink or by partially overlapping the impressions of contiguous dots.
In both cases a large quantity of ink must be deposited on the paper. As a result the drying time increases and the paper tends to warp.
Hence, an acceptable optical density can be obtained only at the expense of both the drying time, which becomes longer, and the flatness of the paper, which tends to become crinkled.
On the other hand, reducing the volume of ink expelled from the single nozzle of each chamber produces on the paper smaller dots separated by larger white zones, causing an even greater deterioration in the optical density.
In order to obtain uniform shades of grey, the optical density must be varied in direct proportion to the number of dots deposited for a given matrix. In practice if a single nozzle is used, the optical density increases in direct proportion to the number of dots deposited in the case of low coverage, for medium coverage (40-75%) it increases more rapidly than the number of dots deposited, while for high coverage it increases less rapidly than the increase in the dots deposited, on account of the random merging of a certain number of adjacent impressions. For example, if the number of dots deposited is increased from about 80% to 100%, from a visual inspection the optical density does not appear to increase.
Finally, the profile of the edge of elongated impressions, especially in the direction perpendicular to the movement of the head, for example in the case of characters l, k, etc., has the appearance of a succession of rounded arches, resulting in a poor print quality.